1. Field of the Invention
The present invention relates to a linear block copolymer. More particularly, the present invention is concerned with a linear block copolymer comprising at least two vinyl aromatic hydrocarbon polymer blocks (S), at least two conjugated diene polymer blocks (B) and at least one vinyl aromatic hydrocarbon/conjugated diene copolymer block (B/S); the linear block copolymer having a specific block configuration wherein both terminal polymer blocks of the linear block copolymer are S blocks, both terminal S blocks have, bonded directly to the respective inner ends thereof, B blocks, and the B blocks, which are bonded directly to the respective inner ends of both terminal polymer blocks (S), have therebetween one or two B/S blocks which is or are bonded directly to the respective inner ends of the polymer blocks (B); the linear block copolymer comprising at least two fractions having different peak molecular weights, and both terminal S blocks in total comprising at least two fractions having different peak molecular weights. The present invention is also concerned with a resin composition comprising the linear block copolymer and a styrene-containing resin in a specific amount ratio. By molding the linear block copolymer of the present invention or the resin composition containing the linear block copolymer, a shaped article retaining high transparency and having excellent rigidity and excellent impact resistance can be obtained.
2. Prior Art
A block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene wherein the vinyl aromatic hydrocarbon content is relatively high, has various excellent characteristics, such as transparency and impact resistance, so that such a block copolymer is used for producing injection-molded products, extrusion-molded products (such as a sheet and a film) and the like. Conventionally, in an attempt to improve the mechanical properties (such as impact resistance) of a shaped article produced from a block copolymer while retaining the transparency of the shaped article, there have been proposed not only block copolymers comprising a vinyl aromatic hydrocarbon and a conjugated diene and methods for producing such block copolymers, but also resin compositions comprising such block copolymers. For example, in an attempt to improve the transparency and impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 52-58788 (corresponding to U.S. Pat. No. 4,167,545) discloses a branched block copolymer obtained by division-wise adding a catalyst to a block copolymerization system. Also, for improving the environmental stress cracking resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 4-277509 (corresponding to U.S. Pat. No. 5,227,419 and European Patent No. 492490) discloses a method for producing a block copolymer having gradually varied properties, which comprises division-wise adding a catalyst to a block copolymerization system. On the other hand, for obtaining a block copolymer having transparency and mechanical properties, Unexamined Japanese Patent Application Laid-Open Specification No. 63-145314 (corresponding to U.S. Pat. No. 4,939,208 and European Patent No. 270515) discloses a method for producing a block copolymer having an S.sub.1 -B.sub.1 -B/S-S.sub.2 block configuration, wherein S.sub.1 and S.sub.2 each represent an aromatic vinyl uniform polymer block, B.sub.1 represents a conjugated diene uniform polymer block, and B/S represents a random copolymer block. Further, with a view to improving the transparency and impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 7-97418 discloses a block copolymer which has characteristic features with respect to the vinyl aromatic hydrocarbon block ratio, the arrangement of the polymer blocks, the ratio of the conjugated diene in a segment in which a vinyl aromatic hydrocarbon and a conjugated diene are randomly copolymerized, and the like. However, the above-mentioned conventional block copolymers have problems in that when such a block copolymer is formulated into a resin composition thereof with a styrene-containing resin and molded to obtain a sheet, followed by processing to produce a shaped article, such as a cup for beverage or a cup for frozen or cold dessert, the shaped article has an unsatisfactory balance of transparency, impact resistance, rigidity and the like. Accordingly, it has been desired to produce a block copolymer which is improved so that it is capable of producing therefrom a shaped article having excellent transparency, impact resistance, rigidity and the like.